1. Field of the Invention
The present invention relates to a thin film transistor (“TFT”) and a method of fabricating the same, and more particularly, to a thin film transistor (“TFT”) in which a leakage current can be effectively reduced and a method of fabricating the same.
2. Description of the Related Art
Polycrystalline silicon (“p-Si”) has higher electron mobility than that of amorphous silicon (“a-Si”), and thus is very useful for use in a switching device for display and memory devices.
It is well known that, in a p-Si transistor, leakage or loss of current occurs due to grain boundary traps in a depletion region of a drain of the p-Si transistor. Hydrogenation, in which the grain boundary traps are reduced by impurity implantation, still allows the grain boundary traps to remain. If a gate voltage and a drain voltage increase by the residual boundary traps, a leakage current still increases.
An offset structure has been proposed to more effectively reduce a leakage current. An offset region is a lightly doped drain (“LDD”) and is located between a channel and a gate or a drain, respectively. The LDD deteriorates an electric field of the drain and reduces field emission caused by a gate voltage and a drain voltage. However, in the prior art, a local differential doping process using a mask is required to form an offset structure. In order to successfully perform differential doping in the offset region using the mask, the mask should be precisely aligned on a substrate. However, since the method uses a separate mask, a doping process is complicated and the method cannot be used in a substrate that is not resistant to heat, for example, a plastic substrate, etc.
Woo-Jin Nam et al., Electrochemical and Solid-State Letters, 8 (2) G41-G43 (2005), suggests elimination of residual ion doping damage from a drain junction by oblique-incidence excimer laser annealing (“OI-ELA”). In order to eliminate the residual ion doping damage in OI-ELA, a scanning laser beam near the drain junction disposed below a gate is directed to be incident on a substrate along an inclined optical axis. Thus, damage to the drain junction can be eliminated. However, since the scanning laser beam is incident to be inclined on the substrate, energy of the laser beam is nonuniformly distributed on the surface of silicon. The nonuniform energy distribution causes nonuniform heat treatment of a source and drain. Therefore, study of the nonuniform energy distribution and a method of solving the problem are desired.